This might help a bit: (took me a while)
[latex]\frac{4\: \mathbf{V}}{\: }* \frac{1000\; mV}{\mathbf{V}}[/latex]
This is basically something you need to learn how to do and it can go on and an to get to the unit you want. So, say by knowing basic conversions, you can convert miles to cm just my using the proper fractions. They can be used upside down if needed. The object in the end is to have the units cancel. I've made the V's in Bold and those unots will cancel, leaving mV as the final unit because I know that 1000 mV = 1V,
So, I "CONVERTED" 4 V to Millivolts by knowing that 1000 mV = 1 V
So, knowing that there are 5280 ft in a mile, 12 inches per foot and 2.54 cm per inch, you can convert miles to cm. Arrange the fractions such that the units cancel.
x miles * 5280 ft/1 mi * 12 inches/1 ft * 2.54 cm / 1 in; The answer will be in cm.
You need to do this will any problem with units. Some units like ampere can be alternatively coulombs/sec.
So, it's showing how to convert 4 or anything written in Volts to mV. Volts is what's know as a basic unit,
EDIT: Added unit conversion example.